The present invention relates to a carbonylation process improvement. More particularly, this invention relates to an improved process scheme wherein at least a portion of the reaction mass from a carbonylation process can be withdrawn from the reactor and separated at a lower pressure from a catalyst-containing stream which is recycled to the reactor. In this processing scheme the catalyst is stabilized in soluble form and any of the catalyst which may have precipitated is reconverted to a souble form.
Recently, processes for producing carboxylic acids and esters by carbonylating olefins, alcohols, esters, halides and ethers in the presence of homogeneous catalyst systems that contain rhodium and halogen components such as iodine components and bromine components have been disclosed and placed into commercial operations. These recently developed processes represent a distinct improvement over the classic carbonylation processes wherein such feed materials have been previously carbonylated in the presence of such catalyst systems as phosphoric acid, phosphates, activated carbon, heavy metal salts and metal carbonyls such as cobalt carbonyl, iron carbonyl and nickel carbonyl. All of these previously known processes require the use of extremely high partial pressures of carbon monoxide. These previously known carbonylation systems also have distinct disadvantages in that they require higher catalyst concentrations, longer reaction times, higher temperatures to obtain substantial reaction and conversion rates that all result in larger and more costly processing equipment and higher manufacturing costs.
The discovery that rhodium and iodine or bromine containing catalyst systems will carbonylate such feed materials as olefins, alcohols and esters, halide or ether derivatives of the alcohols at relatively mild pressure and temperature conditions was a distinct contribution to the carbonylation art. In spite of the vast superiority of these newly developed catalyst systems, it has been found that conventional processing schemes for separation of the carbonylation products from the liquid reaction mass has posted problems of catalyst inactivation and precipitation from carbon monoxide-deficient streams.
It has been disclosed in U.S. Pat. No. 3,845,121 that by withdrawing a portion of the liquid reaction mass from the reactor and passing it to a separation zone of substantially lower pressure, without the addition of heat, at least a portion of the carbonylation products can be vaporized and passed on to purification equipment with much reduced decomposition of the carbonylation catalyst system. According to this scheme, the carbonylation reaction is carried out in the reaction zone at a temperature of from about 50.degree. to about 500.degree. C. and a pressure of from about 345 to about 10340 kPa. By withdrawing a portion of the liquid reaction mass and passing it to a separation zone that is maintained at a pressure that is substantially lower than the pressure in the reactor, at least a portion of the carbonylation products will vaporize with much reduced decomposition of the liquid catalyst system. This vaporization will take place without the addition of heat to the reaction mass. The unvaporized liquid in the separation zone containing the catalyst system can be recycled to the reactor.
Using this processing scheme, it has been found that catalyst precipitation may occur, though to a reduced degree, from liquid streams which are deficient in carbon monoxide. Such streams include the stream of reaction mass withdrawn from the reaction zone in which CO has been consumed by reaction and the liquid cycle stream returned from the separation zone to the reaction zone.
From U.S. Pat. No. 3,818,060 it is known that pentavalent nitrogen and phosphorous compounds of the form XNR.sub.3 or XPR.sub.3 wherein X is oxygen or sulfur may be used as stabilizers for rhodium catalysts in the liquid phase carbonylation of ethylenically unsaturated compounds. Also, from U.S. Pat. No. 3,579,552 it is known that, inter alia, phosphines, amines and trihalostannate compounds form coordination complexes with rhodium and carbon monoxide which remain soluble in the carbonylation of ethylenically unsaturated compounds.
Accordingly, it is an object of this invention to prevent precipitation of the souble catalyst system from CO-deficient streams.
Another object of this invention is to reconvert precipitated components of the catalyst system to soluble form.
Additional objects of this invention will become apparent from the following discussion of the invention.